Heat exchangers for automobile are usually assembled by brazing, using lightweight aluminum alloys as raw materials.
Since it is well known that a heat exchanger for automobile is often used under a severely corrosive condition, the material of the heat exchanger is required to be excellent in corrosion resistance. To solve this problem, corrosion resistance of an aluminum alloy core material has been enhanced, by cladding the aluminum alloy core material with an aluminum alloy skin material (sacrificial anode skin material) having a sacrificial anode effect. As the sacrificial anode skin material having the sacrificial anode effect, one containing Zn, Sn, In, or the like in aluminum in an appropriate amount has been developed.
In the clad material described above, usually, together with the sacrificial anode skin material cladding on one face of the core material, an Al—Si-series alloy filler material is clad on the other face of the core material. It has been developed that a small amount of Zn is contained in the filler material, to give the filler material a sacrificial anode effect, thereby a resulting tube for flowing a refrigerant in which the filler material is utilized is also made to be highly corrosion resistant by this sacrificial corrosion resistant effect.
With respect to external corrosion resistance of a heat exchanger, a potential difference is usually provided between a fin material and the surface of a tube material, thereby the tube is prevented from corrosion by the sacrificial corrosion resistant effect of the fin material.
With respect to the Cu concentration in the aluminum alloy clad material, a concentration gradient is formed in the direction of thickness of the clad sheet, and the Cu concentration gradient is appropriately defined so as to improve external corrosion resistance of the tube.
However, the external corrosion resistance has become insufficient in some cases, even in a heat exchanger equipped with a tube having the sacrificial corrosion resistant effect as described above, or in a heat exchanger equipped with a tube taking advantage of the sacrificial corrosion resistant effect of a fin material as described above. This is conspicuous under current situations in which the thickness of the tube wall is extremely reduced to make the heat exchanger lightweight, particularly in the region where a liquid having a corrosion accelerating property, such as one containing an anti-freeze agent, adheres on the tube.
Such decreased corrosion resistance is caused because grain boundaries are preferentially dissolved due to Si-series compounds precipitated at the grain boundaries, when Si of the filler material on the external surface of the tube material diffuses into the core material. When this preferential dissolving due to the precipitated Si-series compounds invade deep into the tube wall to reach the region in which the sacrificial anode skin material components are diffused into the core material, the resulting reached portion causes pitting corrosion, to lead fetal penetration (through hole) through the tube wall. The sacrificial corrosion resistant effect of the fin material becomes incapable of preventing the tube from corrosion in the situations described above. Further, corrosion cannot be sufficiently suppressed from advancing, even by giving the tube with a corrosion resistant capability, for example, by giving a potential difference by diffusion of Cu in the core material, when the tube wall thickness is thinned to a certain extent.
Accordingly, the corrosion described above should be prevented from invading into the total thickness of the tube wall, to obtain sufficiently high resistance to external corrosion of the heat exchanger when the thickness of the tube wall is required to be as thin as possible.